1. Field of the Invention
This invention concerns a device for reversible optical data storage using polymeric liquid crystals.
2. Discussion of Background
Between the solid crystalline phase and the liquid melt, called isotropic melts below, intermediate phases occur in certain substances that combine characteristics of both the ordered crystalline state and the disordered molten state in structural and dynamic respects. Thus, these phases are actually fluid but they have optical characteristics, for example, that are characteristic of most crystalline substances but also of partially crystalline substances: they are birefringent. For immediately obvious reasons, we speak of intermediate phases (mesophases) or of liquid crystalline phases. These intermediate phases can be obtained by a change of temperature--in this case we speak of thermotropic liquid crystals--or in solution by changes of concentration. Only thermotropic liquids will be considered below. To characterize the ranges of existence of these intermediate phases, the transition temperatures from the crystalline state into the liquid crystalline state and from the liquid crystalline state into that of the isotropic melt (clearing temperature) are generally specified, as determined calorimetrically or by means of a polarizing microscope. Furthermore, if different liquid crystalline states are present, the set of corresponding transition temperatures are specified. The appearance of mesophases is coupled with peculiarities in the molecular geometry. Spherical molecules cannot develop mesophases, but molecules whose shape can be characterized roughly as cylindrical or disc-shaped can do so. The molecules here can be rigid, and the ratio of their maximum to minimum dimensions (for example, cylinder length/cylinder diameter) must clearly exceed a critical value of approximately 3.
The structure of such mesophases is then characterized by the fact that in the simplest case of cylindrical molecules, in the so-called nematic phase, the molecular centers are distributed randomly as in an isotropic melt, while the long axes of the molecules are oriented parallel to one another. This differs from the condition in the isotropic melt, in which the molecular axes are distributed statistically. The consequences are anisotropic mechanical, electrical, and also optical properties. In the cholesteric phase, there is added a continuous helical variation of the direction of orientation of the long molecular axes as an additional ordering principle, which leads to special optical properties, such as strong optical activity or selective reflection. Finally, in the so-called smectic phases there is an additional regular arrangement of the centers of gravity of the molecules in space to supplement the orderly orientation already described that is characteristic for the nematic state, for example, along only one space axis, but in other smectic modifications also along two or even three axes independent of one another. Nevertheless, these phases are fluid. Disc-shaped molecules can develop so-called discotic phases in which either only the normals to the discs are oriented parallel to one another (as in the nematic phase) or in which the discs are arranged in a regular or irregular manner within columns. We speak of columnar structures in this case.
A characteristic parameter of liquid crystalline structures that is very important for application is the orientation ordering parameter, that is a measure of the quality of the orientation ordering. Its value is between 0 for complete disorientation (as in the isotropic melt), and 1 for perfect parallel orientation of all of the long molecular axes.
The wide distribution of liquid crystalline substances in industrial products such as display units in pocket calculators, wristwatches, or digital measuring instruments, results from the characteristic feature that the direction of orientation that can be represented by the so-called director can be changed easily by externally acting electrical, magnetic, or mechanical fields. The changes in the optical characteristics caused by these, in combination with other components such as polarizers, cellular walls, etc., can be used in display elements to display information. The cell walls here serve to protect the liquid mesophases and provide for the necessary macroscopic shape of the liquid crystal film.
It has been found in recent years that it can be beneficial for many areas of application to combine the properties of liquid crystalline phases with those of polymers. The beneficial polymer characteristics here are good mechanical properties, which makes it possible to produce thin, dimensionally stable films from such substances, and the occurrence of a freezing process (glass transition) which makes it possible to preserve a prescribed orientation structure. Specification of the glass temperature Tg, which can be determined calorimetrically, for example, is used to characterize the range of existence of the solid liquid crystalline phase. Above this temperature, the polymer exhibits a viscoelastic state.
Theories on the development of liquid crystalline phases in general and on the development of such phases in polymer systems in particular, as well as experimental findings, shown that the path to the liquid crystalline polymer leads through the use of rigid mesogenic structural units, such as those characteristic of low molecular weight liquid crystals, in combination with flexible spacer groups and flexible chain molecules. Very diverse structures are possible here. The mesogenic groups in the class of sidechain liquid crystals are fastened to a flexible or semiflexible main chain through a flexible spacer, or optionally without this spacer. The mesogenic groups in this case can be cylindrical or disc-shaped. The main chain can also contain mesogenic groups that are separated by flexible units. Copolymers characterized by the fact that different spacers and/or mesogenic groups occur in a polymer can also develop liquid crystalline phases.
Besides these sidechain liquid crystals, main chain polymers also show liquid crystalline phases under certain conditions. The conditions for this are that the chains are either completely made up of rigid groups or of rigid and flexible groups. Copolymers of various mesogenic groups and/or spacer groups can likewise develop liquid crystalline phases. The mesogenic groups can have either a cylindrical shape or the shape of a rod. The nature of the mesophases and the ranges of existence of these phases and of the glassy state can be adjusted approximately through the structure of the mesogenic groups, through the spacer length and flexibility, the flexibility of the main chain, and through its tacticity and length.
Up to now, almost exclusively main chain polymers with exclusively rigid units or with predominantly rigid units have been introduced into the market. They have extremely high values of strength and rigidity. We speak of self-reinforcing thermoplastics. Their fields of use are technical sectors in which extreme mechanical properties are required. (See Kirk-Othmer, Encyclopedia Chemical Technology, 3rd Ed. Vol. 14, pp. 414-421, (1981); J. H. Wendorff, Kunststoffe 73, 524-528 (1983); M. G. Dobb, J. E. McIntyre, Adv. Polym. Sci. 60/61, 61-90 (1984).
Polymers with flexible and rigid units have not yet found use in systems introduced to the market. Their advantage consists of a value of the orientation ordering parameter that is high in comparison with sidechain liquid crystals (See C. Noel, F. Laupretre, C. Friedrich, B. Fagolle, L. Bosio, Polymer 25, 808-814 (1984); B. Wunderlich, I. Grebowicz, Adv. Polymer. Sci. 60/61, 1-60 (1984), Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 14, pp. 414-421 (1981). The polymers with mesogenic side groups have also been given much attention most recently (See S. B. Clough, A. Blumstein & E. C. Hsu, Macromolecules 9, 123 (1976); V. N. Tsekov et al, Europ. Polymer I. 9,481 (1973); L. Strzelecky & L. Libert, Bull. Soc. Chim. France 297 (1973); H. Finkelmann in "Polymer Liquid Crystals", Academic Press, 1982; J. Frenzel, G. Rehage, Macromol. Chem. 814, 1689-1703 (1983); Macromol. Chem. Rapid Commun. 1, 129 (1980); D. Hoppner, J. H. Wendorff, Die Angewandte Makromolekulare Chemie 125, 37-51 (1984), DE-A 27 22 589, DE-A 28 31 909, DE-A 30 20 645, DE-A 30 27 757, DE-A 32 11 400; EU-A 90 282.
U.S. 4,293,435 discloses a technical use of the specific behavior of liquid crystalline polymers that is associated with the transition into the glassy state. In this case, information is stored by the application of conditions that change the arrangement and orientation of the liquid crystalline polymers in a definite manner (for example, electrical and magnetic field or pressure). This state of the art is discussed in British Pat. No. 2,146,787. It is pointed out that the storage of the device provided for in U.S. Pat. No. 4,293,435 in the solid state below the glass temperature (Tg) means that Tg is above ordinary room temperature (Ta), i.e., that the polymer system is used at temperatures that are of the order of magnitude of 100.degree. C. above Ta if it is desired to record the information within a reasonable time. Such temperatures are said to be inconvenient and in the longer view would involve a degradation of the polymer. These difficulties can be avoided according to the British Patent if certain polymeric sidechain liquid crystals are used. It is then no longer necessary to keep the temperature below the Tg to store the device, but stable storage for many years should be possible with temperatures above Tg and below a temperature (Tf) at which the polymeric material begins to become fluid.
The determination of the Tf can be accomplished by following the transmission of light through a liquid crystalline polymer between two crossed polarizing filters with the temperature increasing from the glass temperature. Several degrees below the smectic-isotropic phase transition, the optical transmission suddenly increases. This increase originates from the transition of an anisotropic but almost opaque state to a highly birefringent, transparent state of the range. The temperature range above this temperature Tf is called the "fluid region". The optical transmission also increases with increasing temperature until it reaches a maximum at a temperature Tm. The Tm marks the point at which the isotropic (clear) phase first occurs.
Since the occurrence of the isotropic phase leads to extinction of the light with crossed polarizers, a further temperature increase brings about a decrease of the light transmission to the extent that the isotropic regions increase in size, until the so-called clearing temperature (Tc) is reached, at which the last residues of the structure responsible for the birefringence have disappeared.
British Pat. No. 2,146,787 claims a device with a material film that contains a liquid crystalline polymer with mesogenic sidechains, and devices for the thermal conversion of at least a portion of the material from the viscous state in which the temperature of the material is in the region of Tg to Tf, into the liquid range, and devices for controlling at least a portion of the material in the liquid region, by which a selective change in the texture of the molecules in the material is produced. Information is thereby input that is retained after the cooling of the liquid region and its return to the viscous state. It si thus an essential prerequisite for the British Patent to use polymer material for which it is true that Tf&gt;Ta&gt;Tg. A device is also described in which the material film contains a liquid crystalline polymer with a smectogenic sidechain. Polymeric liquid crystals of the polysiloxane type with diphenylcyano sidechains or benzoic acid ester sidechains are particularly preferred.
Now as ever, there is great interest in optical storage media that are capable of reversible storage in addition to high display densities. The methods described above for solving the problem of optical data storage represent relatively narrow, limited technical solutions. Thus, the device of British Pat. No. 2,146,787 depends on the use of liquid crystalline sidechain polymers with the essential prerequisite that the temperature is selected so that the polymeric material is kept in a viscous state region. The disclosure covers polysiloxane liquid crystals, preferably with diphenylcyano or benzoic acid ester sidechains. The stability of the stored information is not absolutely guaranteed because of the molecular mobility that is present and the finite relaxation times, and also because of the possibility of effects on the system of interfering fields, for example. Furthermore, technical methods that could be carried out without an excessively narrow latitude would be desirable.
The German Pat. applications P 36 03 266.2, P 36 03 267.0, and P 36 03 168.9 describe the storage of optical information in liquid crystalline main chain polymers and sidechain polymers. The entry of information here occurs through local heating in the isotropic phase or through reorientation of the molecules in the anisotropic phase. These writing processes require relatively high intensities of the writing laser.